Myocardial leads have electrodes which are attached to the exterior of a human heart and deliver stimulation pulses from an implanted cardiac pacemaker to a patient's heart. The myocardial leads are attached to the heart typically by positively fixating a distal electrode into the myocardial tissue. A skirt portion located behind the electrode typically is used for passive tissue ingrowth. Generally, it is preferable to use two myocardial leads, for stimulating in a bipolar fashion, to prevent inadvertent stimulation of skeletal muscle tissue or other tissue during cardiac pacing.
One such myocardial lead is discussed in U.S. Pat. No. 5,154,183 (Kreyenhagen et al.), which reference is hereby incorporated herein by reference. The myocardial lead includes a center section and a number of leaf-shaped appendages, or petals, extending outwardly from the center section. The center section has a positive fixation electrode made of electrically conductive material. The leaf-shaped appendages are securely connected to the center section and extend radially outward from the center section.
Each leaf-shaped appendage includes at least one conductive segment. The conductive segments on the leaf-shaped appendages are also electrically insulated from the first member on the center section. The leaf-shaped appendages conform to the heart's surface, flex to insure reliable contact with the heart tissue, and will maintain contact during both heart contractions and rest periods. The leaf-shaped appendages may be electrically connected together to act as a second electrode for bipolar sensing, or may be individually used as separate electrodes in a multipolar fashion.
Many types of implantable systems are known for tachyarrhythmia control. Such systems have gained greater acceptance in recent years as an alternative therapy to chronic pharmacologic treatment. Such tachyarrhythmia control systems typically include an implantable device capable of tachyarrhythmia detection and delivery of an automatic therapeutic response to the arrhythmia, including bradycardia pacing support, anti-tachyarrhythmia pacing, low energy synchronized cardioversion or high energy defibrillation shock, an electrode system for sensing and pacing, and a high energy electrode system for delivery of defibrillation shock. Typically the pacing and sensing electrode system will consist of a bipolar endocardial lead or two unipolar myocardial leads. The high energy electrode system generally consists of two myocardial patch leads, or a transvenous shocking lead with either a myocardial or subcutaneous patch lead.
Any device that is intended to provide automatic treatment of ventricular tachyarrhythmias must be capable of first detecting the presence of such arrhythmias prior to the onset of therapy. Several methods are known for detecting ventricular tachyarrhythmias. These include monitoring an absolute heart rate interval, and initiating therapy when the interval becomes less than a programmable interval threshold.
It is also known to try to differentiate pathologic rhythms from normal physiologic rhythms by analyzing the rate of onset (sudden change, as opposed to gradual change in the heart rate interval) and/or heart rate stability.
It is also known to determine the probability density function of a signal corresponding to heart activity, which involves the evaluation of the time that the cardiac electrical signal spends at an isoelectric base line, and to initiate therapy when deviations beyond a predetermined threshold occur.
The known detection techniques have several limitations and disadvantages. The two major disadvantages are (1) no accurate method of differentiating between a pathologic (i.e. hemodynamically compromising) rhythm versus a physiologic (i.e., sinus) rhythm, and (2) total reliance on a processed electrogram for detection of cardiac depolarization. As a result of the second disadvantage, certain rhythms, particularly low amplitude ventricular fibrillation, may not be detected. These limitations in the known detection techniques may result either in a false positive detection response (inappropriate shock delivery) or a false negative detection response (failure to respond to a pathologic rhythm).